"Even though trains have traveled over 200 mph under test conditions, it is still a very ticklish business to run regular passenger service at speeds greater than 150 mph.
Many of the problems of the railroad can be traced to the high concentrations of force which occur at the point of contact with the rail. The Tracked Air Cushion Vehicle avoids this by spreading the suspension force over a large area. Pressurized air is fed into a cushion region, from which it escapes through a small gap between the cushion lip and the guideway.
Henry Kohm and Richard Thornton of MIT have described a vehicle which utilizes magnetic repulsion to levitate at speeds of 300 mph or greater. The vehicle uses wheels at low speeds, and the guideway is shaped like a semi-circular trough so the vehicle can bank like a toboggan through turns.
The large magnetic field which is the essential feature of this concept is not without its difficulties. Some form of magnetic shielding is necessary to protect the passengers, and steel in the guideway causes problems because of the resulting attractive force at low speeds. Concrete also introduces difficulties since it enters a corrosive reaction with the aluminum levitation surface. The civil engineers designing the guideway must therefore work with special considerations regarding building materials, which is bound to increase the eventual cost.
The only advantage to magnetic repulsion is that it can operate in an evacuated tube. However, the difficulties of such an operation are formidable. Rather dramatic and unforeseen reductions in the cost of tunneling and maintaining an evacuated environment are required before investments in this concept of transportation can be justified.
The probability is quite high that the suspension systems mentioned here are technically feasible. Economic feasibility, however, is the more important question and the one which will eventually decide which scheme is preferable. Determination of this issue will be one of the more fascinating technological, sociological, and economic topics of the next decade.”
Excerpted from “Suspension Concepts for High-Speed Ground Transportation,” by Timothy M. Barrows of the U.S. Department of Transportation, from the July/August 1975 issue of Technology Review.